Abstract Manganese (Mn) is an established neurotoxicant that affects motor and cognitive brain pathways. This proposal builds on a growing body of Mn neurotoxicity research generated by our investigative team and others. Despite these important contributions to studying the health effects of Mn exposure in vivo, defining the associated neuropathology is essential to understand mechanisms of injury and to characterize dose- response relations that will inform regulatory policy. These types of studies are extremely challenging due to the difficulty in acquiring human brain tissue and quantifying lifetime exposure to Mn in the same subjects. Over the last seven years, we have developed a collaboration with the University of the Witwatersrand in Johannesburg, South Africa using the only population-wide, occupational autopsy program in the world. Through this collaboration we have conducted novel preliminary Mn neuropathology studies that support the aims in this proposal. Our data suggest that chronic, low-level Mn exposure in these mines is associated with lower neuronal density and higher microglial/astrocyte ratios in the caudate and putamen, indicating that Mn exposure may cause astrocytic dysfunction which in turn induces a pro-inflammatory neurotoxic state in the corpus striatum (caudate, putamen, globus pallidus), driven by the activation of microglia. Astrocytic dysfunction is likely attributable, in part, to dysregulation of several key mitochondrial proteins induced by Mn exposure. However, our preliminary studies also suggest that Mn mineworkers, with high MRI signal intensity on T1 MRI, have similar corpus striatal tissue Mn concentrations, but possibly lower Fe concentrations, than non-Mn mineworkers. In this proposal, we will follow-up on these preliminary data by collecting brains from deceased Mn mineworkers with contemporaneous Mn exposures and appropriately matched non-Mn mineworkers. We will use unbiased stereologic methods to quantify neurons, astrocytes, and microglia in the caudate, putamen, globus pallidus, substantia nigra pars compacta (SNpc), and olfactory bulbs in both groups of workers and investigate the overall and dose-response associations between these counts and cumulative Mn exposure. We will also use immunofluorescence microscopy to quantify targeted astrocytic mitochondrial proteins and inductively coupled plasma-mass spectrometry to compare corpus striatal Mn and Fe concentrations between groups. This highly innovative study will provide a rare opportunity to advance the field of Mn neurotoxicity by investigating the neuropathologic effects of chronic Mn exposure.